EP-A1-0 016 895 reveals a heavy-duty dishwasher of this type, which contains in its tank a granule collecting sieve container with its inlet connected to the washing chamber and its outlet connected to the suction chamber of the pump. A gate closes in one end position the outlet of the sieve container and thereby opens a connection between the suction chamber and portions of the tank lying outside the sieve container, so that only liquid is sucked up by the pump: In the other end position of the gate, said connection is closed and the pump sucks liquid from the tank via the sieve container, whereby the liquid sucked in has granules from the sieve container. In addition to the fact that such a dishwasher has relatively complicated construction, it requires time consuming maintenance in order to prevent the sieve container from being clogged by food particles carried by the circulating liquid.
EP-A2-0 195 959 and EP-A1-0 169 847 show dishwashers of the above-mentioned type for granules heavier than the washing liquid. In such machines, a vertically moveable cylinder is disposed at the bottom outlet of the tank. In the upper position of the cylinder, liquid and granules flow from the tank through the annular gap between the lower end of the cylinder and the bottom outlet of the tank. This gap is closed when the cylinder is in its lower position, where liquid from the tank is sucked in through a gap between the upper end of the cylinder and a conical plate which prevents granules from flowing from above into the upper open end of the cylinder. The dishwasher according to EP-A2-0 195 959 also contains an annular sieve plate in the last mentioned gap in order to prevent granules from accompanying liquid sucked into the upper end of the cylinder. In EP-A1-0 169 847, it is assumed that the granules will sink by their own weight to the bottom of the tank, and that only liquid will be sucked into the upper end of the cylinder when the cylinder assumes its lower end position. However, granules which pass by the upper end of the cylinder when it serves as a pump inlet will be sucked into the inlet, and therefore it takes a relatively long time for such a machine to assume a state where only liquid is circulated and all the granules have sunk to the bottom of the tank.